Question

Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ‘plum pudding model’ of the atom?

Answer 1

In Thomson’s model, the positive charge is spread evenly over the whole atom, so its electric field is too weak to push a fast, heavy α-particle straight back. A particle bouncing back needs a very large force, which can only come from a tiny, dense concentration of positive charge. This is why the backscattering rules out Thomson's model, which predicts only slight deflections.